A surface protection film is a pressure-sensitive adhesive plastic film for protecting a product such as plastic film, resin plate, metal foil, metal plate, or the like (hereinafter referred to the adherend) from scratches and contamination in the production process or transportation which is used by adhesion onto the surface of the adherend.
The surface protection film is required to retain the minimum necessary pressure-sensitive adhesion function under the conditions of use while it is also required to be easily peelable once the surface protection function is no longer needed. Simultaneously, the surface protection film should not leave any mark of its use such as smudge and scratch on the adherend. In particular, when the surface protection film is used for an optical film such as a phase difference film used in liquid crystal display, there is a strict requirement that the so-called fish eye which is the foreign matter in the surface protection film or the protrusion formed by the oxidized resin should not be transferred to the adherend as dents. The requirement is increasingly strict in these years, and such transfer defects should not be generated by the increased internal pressure due to the tension of the film itself even when the surface protection film is adhered to the adherend, wound in the roll form, and stored for a long term.
With regard to the reduction of the fish eyes, Patent Document 1 discloses a method wherein a molten resin ejected from the T die is pressed between a metal roller having a smooth surface (with the maximum height Ry of up to 0.5 μm) and a touch roller having an elastic smooth surface (with the maximum height Ry of up to 0.5 μm) to crush the fish eye, although this document is not related to a surface protection film.
However, due to the pressure-sensitive adhesive nature of the surface protection film, the surface protection film wound in roll form often suffered from the problem of blocking, namely, adhesion of the surface having the pressure-sensitive adhesive force (hereinafter referred to as the “adhesive surface”) to the other surface of the film (hereinafter referred to as the “rear surface”). Accordingly, even if all fish eyes could be crushed by the use of such technology of the Patent Document 1, use of such technology alone still resulted in the smooth opposite surfaces of the surface protection film and even severer blocking problem, and the wound film could not be rewound and use of such surface protection film was practically unacceptable.
As a countermeasure for such blocking problem, there has been proposed a method wherein the surface of the nip roller is formed from a rubber so that the rear surface would a surface like a ground glass or an embossed surface (for example, Patent Documents 2 and 3).
In this method, the surface morphology of the rubber covering the nip roller is transferred to the rear surface of the film, and the rear surface of the film will be provided with the surface irregularities like that of the ground glass. The thus introduced cushioning effect results air entrapment between the layers, and releasability is thereby provided. This releasability prevents the blocking. Furthermore, the resulting slipperiness contributes for the good roll shape.
However, it is the finding of the inventors of the present invention that, this technique has the problem that, when the adherend is a very soft film as in the case of the phase difference film, the surface irregularity morphology of the rear surface is transferred to the adherend during the long term storage after adhering with the adherend and winding in the form of a roll, and in such a case, the adherend having the resulting dents is no longer usable.
Patent Document 4 also suffers from the problem that, since transfer of the surface irregularity morphology to the adherend is not at all considered despite limitation of the arithmetic mean roughness Ra of the nip roller surface and the surface of the film having the surface irregularity to the range of 0.8 to 10 μm as a countermeasure to the blocking, the surface irregularity morphology of the rear surface is highly likely to be transferred to the adherend surface in the form of dents as described above when the Ra is 1 μm or higher, and depending on the surface irregularity morphology, the transfer of the surface irregularity morphology even still occurs even if Ra were less than 1 μm. This is the result of the fact that the arithmetic mean roughness Ra is a parameter which is less likely to reflect the morphology of the less frequent and relatively large surface irregularities and the density of the surface irregularities. More specifically, the inventors of the present invention have found that the transfer of the surface irregularity morphology to the adherend is affected by the size and density of the surface irregularities, and prevention of the transfer of the surface irregularity morphology to the adherend is difficult by solely considering the arithmetic mean roughness Ra.
The inventors of the present invention also proposes a method wherein a compactly embossed surface is formed on the surface of a multi-layer surface protection film by forming the layer constituting the rear surface (hereinafter referred to as the rear surface layer) from a polypropylene resin and a polyethylene resin without the step of pressing the film between the cooling roller and the nip roller (Patent Document 5). In this method, however, the use of the polypropylene resin resulted in the excessive hardness of the rear surface, and accordingly, when such surface protection film is adhered to a very soft adherend such as a phase difference film and stored for a long time after winding in roll form, complete prevention of the transfer of the surface irregularity morphology of the rear surface to the adherend surface was sometimes impossible.